To assist tracking soil temperature, the MDA provides real-time soil temperatures at 48 locations across the state at https://app.gisdata.mn.gov/mda-soiltemp/. The website includes a map with MDA sites with soil thermometers at a six-inch depth, North Dakota Ag Weather Network sites at four-inch depths, and research sites at various depths.

“There are areas of the state where fall application of nitrogen fertilizer is simply not recommended due to groundwater contamination concerns,” said Bruce Montgomery, manager of the MDA Fertilizer Management Section. “Those would be areas with coarse-textured soils that drain quickly or areas underlain by fractured bedrock karst geology. In other areas of the state where fall nitrogen fertilizer application is a recommended practice, the MDA encourages delaying application until soil temperatures cool down.”

Map showing average date that 50 degree F soil temperature is reached in Minnesota. On average soil temperatures reach 50 degrees F during the first week in October in northern Minnesota and the fourth week of October in southern Minnesota.

Waiting until soil temperature stays below 50 degrees F before applying anhydrous ammonia and urea increases the availability of nitrogen to next season’s crop and decreases the amount of nitrate that could potentially leach into groundwater or tile drainage. At cooler temperatures microbial activity in the soil slows down, slowing the conversion from ammonium to nitrate. Ammonium is stable in the soil whereas nitrate moves with water and may leach out of the root zone over winter and early spring.

Although the soil temperature network was established to support application of commercial fertilizer it is equally useful for those applying manure in the fall. University of Minnesota Extension recommends delaying fall manure applications until soil temperatures at six-inch depth are below 50 degrees F to prevent leaching losses. Research from the University of Minnesota at Waseca showed liquid dairy and hog manures injected in November produced yields 10 bushels per acre higher than manures injected in September and October.

In addition to delaying application until soil temperature stays below 50 degrees F, best management practices for nitrogen use developed by the University of Minnesota Extension for south-central Minnesota recommend using a nitrification inhibitor when fall applying anhydrous ammonia and not to apply urea in the fall. In drier western Minnesota fall application of both anhydrous ammonia and urea are recommended practices. In southeast Minnesota’s karst region and statewide on coarse-textured soils, fall application of nitrogen fertilizer is not recommended regardless of soil temperature. Fall application of 28% liquid nitrogen is not recommended anywhere in the state due to its high leaching potential. Specific nitrogen fertilizer use recommendations by region of the state can be found at www.mda.state.mn.us/nitrogenbmps.

The MDA is proposing a rule that by 2019 may restrict fall nitrogen fertilizer application in areas vulnerable to groundwater contamination. The rulemaking is part of the state’s Nitrogen Fertilizer Management Plan. More information on the proposed rule can be found at www.mda.state.mn.us/nfr.

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